Infrared absorption in the Drude tail of doped semiconductors: The effect of finite electron densities

We discuss the influence of finite electron densities and electron-electron interactions on the infrared absorption coefficient of doped (n type) semiconductors, for frequencies where the absorption coefficient is dominated by the free-carrier absorption. The aim is to extend earlier theories of LO-phonon-induced free-carrier absorption up to carrier densities where the electron plasma frequency is comparable to or higher than the LO-phonon frequency. Numerical studies of our expression for the absorption constant show that the coupling of the LO phonon to the electron plasma produces only small corrections to the absorption coefficient, for parameters appropriate to typical semiconductors. However, there are single-particle effects evident when Ω the Fermi energy EF. In this regime, we find deviations from the predictions of earlier theories valid only for low carrier concentrations.